It is desired that solar cells are put to practical use, as clean energy sources which are safe and give no bad effect on the environment, within a wider scope.
FIG. 1 is a view which schematically illustrates an ordinary sectional structure of a solar cell element.
As illustrated in FIG. 1, an n type impurity layer 2 having a thickness of 0.3 to 0.5 μm, an antireflection layer 3, and grid electrodes 4 are successively formed on the side of a light-receiving face of a p type silicon semiconductor substrate 1 having a thickness of 300 to 600 μm.
An aluminum electrode layer 8 is formed as a backside electrode on the backside of the p type silicon semiconductor substrate 1. The aluminum electrode layer 8 is composed of an aluminum sintered layer 5 and an aluminum silicon mixed layer 6, and is formed by applying a paste composition comprising aluminum powder, glass frit, and an organic vehicle by screen printing or the like, drying the composition, and then firing the composition at a temperature of 660° C. (the melting point of aluminum) or higher. At the time of this firing, aluminum diffuses into the p type silicon semiconductor substrate 1, thereby forming the aluminum silicon mixed layer 6 between the aluminum sintered layer 5 and p type silicon semiconductor substrate 1 and simultaneously forming a p+ layer (or p++ layer) 7 as a diffused layer based on the diffusion of aluminum atoms. By the presence of this p+ layer 7, the BSF (back surface field) effect of improving the efficiency of collecting generated carries is produced. Actually, the backside electrode is used in the form of the following cases: the case that the three layers of the aluminum sintered layer 5, the aluminum silicon mixed layer 6, and the p+ layer 7 that are fired are used as they are; and the case that the aluminum sintered layer 5, which is fired, or the aluminum sintered layer 5 and the aluminum silicon mixed layer 6 that are fired are removed by a chemical method or the like in order to decrease the electric resistance, and then an electrode layer made of silver or copper is formed on the surface and used. In any one of the cases, the p+ layer 7 exhibits the BSF effect.
The paste composition used to form the aluminum electrode layer 8 as the backside electrode generally comprises aluminum power, glass frit, and an organic vehicle.
The aluminum powder is added in an amount of about 60 to 80% by mass in order to form the aluminum sintered layer 5, the aluminum silicon mixed layer 6, and the p+ layer 7 based on the diffusion of aluminum atoms.
The glass frit is added in an amount of about 1 to 5% by mass in order to strengthen the bond between the aluminum electrode layer 8 and the p type silicon semiconductor substrate 1 by use of the effect that the frit is melted without being volatilized at the time of the firing and is re-solidified when the frit is cooled.
The organic vehicle is added in an amount of about 15 to 40% by weight in order to improve the applicability or printability of the paste composition when the composition is applied by screen printing or the like.
Since attention is paid to solar cells as clean energy sources which are safe and give no bad effect on the environment, one theme for putting the solar cells to practical use within a wide scope is to remove or decrease materials giving bad effect on the environment from constituents of the solar cells.
Incidentally, paste compositions are variously investigated in order to keep desired functions for the backside electrode of a solar cell while preventing the p type silicon semiconductor substrate from being warped or cracked at the time of firing.
Japanese Unexamined Patent Publication No. 2000-90734 (Patent Document 1) discloses an electroconductive paste containing an aluminum-containing organic compound besides aluminum powder, glass frit and an organic vehicle as an electroconductive paste capable of keeping desired functions for the backside electrode of a solar cell, and making an aluminum electrode layer thin in order to prevent the p type silicon semiconductor substrate from being warped or cracked at the time of firing.
Japanese Unexamined Patent Publication No. 2003-223813 (Patent Document 2) discloses a paste composition which contains, besides aluminum powder and an organic vehicle, an inorganic compound wherein the thermal expansion coefficient is smaller than that of aluminum and any one of the melting temperature, the softening temperature and the decomposition temperature is higher than the melting point of aluminum as a paste composition which makes it possible to attain a desired BSF effect sufficiently without decreasing the coating amount even if a p type silicon semiconductor substrate is made thin, and further restrain the deformation of the p type silicon semiconductor substrate fired.
Furthermore, Japanese Unexamined Patent Publication No. 2004-134775 (Patent Document 3) discloses an electroconductive paste composition containing aluminum powder, glass frit and an organic vehicle and further containing at least one species out of organic compound particles slightly-soluble or insoluble in the organic vehicle and carbon particles as an electroconductive paste composition for keeping a desired function for a backside electrode of a solar cell while making the shrinkage of an aluminum electrode layer small at the time of firing, thereby preventing the generation of warps or cracks.
However, in the paste compositions disclosed in these publications also, glass frit is used as an essential component in the present situation. Hitherto, lead-containing glass, which gives a bad effect on the environment, has been used as the glass frit.
Thus, the use of a lead-free glass such as Bi2O3—SiO2—B2O3 based or ZnO based glass, instead of a lead-containing glass such as SiO2—PbO2 based or SiO2—B2O3—PbO based glass, has been investigated from the viewpoint of not giving any bad effect on the environment.
However, the lead-free glass has problems that the glass has a high melting point, and thus in order to melt the glass homogeneously, the firing temperature rises and the firing time becomes long.
Patent Document 1: Japanese Unexamined Patent Publication No. 2000-90734
Patent Document 2: Japanese Unexamined Patent Publication No. 2003-223813
Patent Document 3: Japanese Unexamined Patent Publication No. 2004-134775